The invention relates to the control of `U` tubing in the flow of cement or other fluids in oil well casings.
As an oil well is drilled, casings of successively decreasing diameters are inserted into the drilled hole, with the final casing, the production casing, conveying the oil from the well to the well head. The succession of casings are cemented in position to, for example, prevent drilling fluid from circulating outside the casing and causing erosion. Cementing is also necessary in the casings close to the surface to seal off and protect fresh water formations, provide a mounting for blow-out preventer equipment and for supporting the inner casings.
Cementing is achieved by preparing a cement slurry and then pumping it down the casing. As it is pumped down, the cement slurry displaces the mud already in the casing and passes out of the end of the casing and then up the exterior of the casing, displacing the mud in front of it. When all the mud has been displaced and the cement slurry is therefore continuous around the outside of the casing, pumping stops and the cement is allowed to set. The end of the casing includes a one-way value which, when cementing is complete, prevents the cement passing back up the casing.
The cement slurry has a density which is greater than the density of the mud which it displaces. This can result in the phenomenon of `U` tubing in which the forces resisting the flow of cement are insufficient to allow the pumping pressure to be maintained and the cement slurry falls in the casing under the effect of gravity faster than the pumping rate. Accordingly, when `U` tubing occurs, the cement slurry is no longer under the control of the pump.
This is undesirable because the increased flow rates in `U` tubing can cause a strongly turbulent flow which can erode seriously any weak formations around the casing and cause laminar flow an undesirable flow regime while equilibrum is being sought. Further, it can result in a vacuum being formed behind the `U` tubing cement slurry and the slurry may then halt while the pump slurry fills the vacuum. It can also cause surging in the rate at which the mud is forced to the surface and this can be difficult to control at surface without causing unfavourable pressure increases downhole.